As pneumatic tires capable of running flat-out, a pneumatic tire, whose side walls are provided respectively with reinforcement rubber layers having a crescent-shaped cross section, which inhibits the tire from being flexed vertically on the basis of the reinforcement rubber layers when the tire is flat, and whose tire casing is accordingly prevented from being destroyed is known.
In a case, however, where the side walls are provided respectively with the crescent-shaped reinforcement rubber layers, this brings about the following disadvantages. The tire increases in weight to a large extent in comparison with regular tires. In addition, an increase in vertical springs makes the riding comfort worse. Furthermore, in a case where the side walls are provided with the respective crescent-shaped reinforcement rubber layers, the center portion of the tread part becomes depressed inwards in the radial direction of the tire under run-flat driving conditions, thus causing a buckle phenomenon. This increases load on each of shoulder parts. This is a factor of reducing the durability of the tire.
Against this background, the followings have been proposed with regard to the tires capable of running flat-out (see Japanese patent application Kokai publication No. Hei 10-151917, for example). A carcass layer is formed of three or more carcass plies. A side reinforcement rubber layer having a crescent-shaped cross section is arranged between each neighboring two of the carcass plies in each of the side wall parts. In addition, a tread reinforcement rubber layer is arranged between each neighboring two of the carcass plies in the tread part.
In the case of such a pneumatic tire, the interposition of the tread reinforcement rubber layer and the side reinforcement rubber layers between each neighboring two of the carcass plies increases bending rigidities respectively in the tread part and the side wall parts, accordingly increasing a mileage in which the motor vehicle can continue running flat-out. Furthermore, if the side reinforcement rubber layers were made thinner, the riding comfort can be improved.
However, in the case of the pneumatic tire for which three or more carcass plies are used, effects of reinforcement which would otherwise be brought about by the carcass plies are not utilized fully. For this reason, if effects of further improved durability are intended to be obtained, the weight is inevitably increased to a large extent.